1. Field of the Invention
The present invention relates to an ink set for color ink-jet recording, ink-jet recording method, recording unit, ink-cartridge, ink-jet recording apparatus and bleed-alleviation method, to obtain color images of high density, grade, sharpness and quality by the ink-jet recording even on plain paper.
2. Related Background Art
Heretofore, in order to form a black image which is excellent in optical density, printing grade, water resistance, and lightfastness on plain paper by the ink-jet recording system, there has been proposed use of black pigment ink. Besides, there have been proposed an ink set which would not cause bleeding at the boundary between an image printed with black ink and an image printed with color ink, as well as the ink-jet recording method or apparatus using it. For example, in Japanese Patent Application Laid-Open No. 10-183046, there is described an ink set capable of reducing bleeding by combination use of a black ink containing a carbon-black pigment having a cationic group on its surface as a coloring material, and a color ink containing an anionic dye as the color material.
A large number of means for improving the optical density and water resistance of recorded images have been proposed to date. As one of the solutions thereof, it is proposed to prepare an ink containing a pigment dispersed in water as a coloring material. For example, Japanese Patent Application Laid-Open No. 8-239610 discloses an aqueous pigment composition for ink-jet recording containing colored resin and a humectant as essential components. As such an ink, there is an ink using carbon black as a coloring material. Such an ink can give images of high optical density and excellent water resistance. However, even such recorded images still need improvement in rub-off resistance and resistance to line marker, particularly on plain paper. However, in the method as described in Japanese Patent Application Laid-Open No. 7-145336, where bleeding in multicolor printing is suppressed by mutual contact between an anionic ink and a cationic ink in the presence of a polymer, the polymer may adversely affect the ink reliability depending on the polymer type. This poor reliability means, if there is a certain time interval, one minutes, for example, between the ink ejection performance from a nozzle in recording, it may happen that the next ink droplet cannot be ejected steadily, causing disturbed printing (such a phenomenon is called xe2x80x9cpoor ejectabilityxe2x80x9d).
Besides, the method described in the above Japanese Patent Application Laid-Open No. 10-183046 uses a combination of a black ink using cationic carbon black and a color ink using an anionic dye, and the aggregates formed between the coloring materials suppress bleeding. However, with the ink-jet inks of which coloring material concentrations are not so high as to cause sufficient aggregation, there may happen, especially on the recording paper of high permeability such as normal paper, bleeding between black and color images, or a phenomenon called xe2x80x9chazexe2x80x9d in the black image. Here, haze, a whitish black region which should be solid black, is caused as the black ink is drawn deep into the paper by the action of highly penetrable color ink in the boundary region between a color image and a black image, when the black ink and the color ink are applied in the same scan.
According to the study by the present inventors on the above mentioned ink-jet ink containing a pigment, colored resin and a humectant as the essential components, the upper limit for each component is automatically determined when the ink ejection stability is considered. Thus, the amount of the pigment which determines the image density and the amount of the colored resin which determines the image fixability must be selected arbitrary considering the balance between the image density and image fastness. Accordingly, the conventional pigment ink is still not making the best use of pigment as a coloring material.
More specifically, Japanese Patent Application Laid-Open No. 8-239610 explains in the specification thereof that the colored resin is a dispersion of a resin colored by a dye. According to the preparation process of the colored resin described in the Example thereof, a dye is first added to an emulsion of a resin, and the mixture is heated to about 80xc2x0 C. and then cooled, thereby preparing the colored resin. However, in page 4, left column, lines 38 to 41 of this patent specification, it is described that xe2x80x9cIn order for the dye to be sufficiently taken in the resin, the amount (of the dye) is preferably 10 parts or less, particularly 8 parts or less, per 100 parts of the resinxe2x80x9d. In each of the preparation processes described in Preparation Examples 8 to 13 of the specification, a mixing proportion of the dye to the solid resin content in the emulsion polymer is described as about 1:10 to 1:12 (dye:resin).
According to the investigation by the present inventors, however, it has been concluded that when such a colored resin as described in Japanese Patent Application Laid-Open No. 8-239610 is used, sometimes such a proportion of the dye to the colored resin as described in the specification is not sufficient. That is, when a colored resin is added to the ink to enhance the fixability of the pigment ink to the recording medium, the amount of the pigment which can be included in the ink becomes low. If the colored resin is added more to the ink to compensate for the reduction in optical density due to the reduction of amount of the pigment or, further, to obtain sufficient image density, it is inferred that the resin content may exceed the range which enables precise ejection of the ink by an ink-jet recording system.
As described above, from the investigations as to the prior art, the present inventors have reached a conclusion that the development of a new technique entirely different from the conventional techniques is required for further improvements in optical density and image fixing ability in pigment inks.
Accordingly, it is an object of the present invention to provide an ink set of black ink and color ink for ink jet recording, which can provide images of high optical density and high print grade as required for black image, and color images of images being excellent in rub-off resistance, water resistance and resistance to line marker in comparison with conventional pigment ink, which is excellent in reliability as the ink-jet ink such as storage stability, wettability to the head face, clogging resistance, and printing durability, and moreover which is highly effective in alleviating bleeding when used in a multi-color printing. The other object of the present invention is to provide an ink-jet recording method, and ink-jet recording apparatus which can achieve above mentioned advantages.
According to one embodiment of the present invention, there is provided an ink set for recording a color image on a recording medium by using ink of two or more colors comprising a black ink and a color ink, wherein the black ink comprises a first pigment having a cationic group or a pigment and a pigment dispersant having a cationic group, and a resin encapsulating a coloring material, and the color ink comprises an anionic dye and an anionic substance.
According to another embodiment of the present invention, there is provided an ink jet recording method comprising the steps of:
ejecting a black ink from an orifice in response to a recording signal to attach the black ink on a recording medium; and
ejecting a color ink from a orifice in response to a recording signal to attach the color ink on the recording medium,
wherein the black ink comprises a pigment having a cationic group or a pigment and a pigment-dispersant having a cationic group, and a resin encapsulating a coloring material, and the color ink comprises an anionic dye and an anionic substance.
According to a further embodiment of the present invention, there is provided a recording unit comprising:
a first ink container containing a black ink;
a second ink container containing a color ink; and
a head for ejecting the black ink contained in the first ink container and the color ink contained in the second ink container,
wherein the black ink comprises a pigment having a cationic group or a pigment and a pigment-dispersant having a cationic group, and a resin encapsulating a coloring material, and the color ink comprises an anionic dye and an anionic substance.
According to a still further embodiment of the present invention, there is provided an ink cartridge comprising:
a first ink container containing a black ink; and
a second ink container containing a color ink;
wherein the black ink comprises a pigment having a cationic group or a pigment and a pigment-dispersant having a cationic group, and a resin encapsulating a coloring material, and the color ink comprises an anionic dye and an anionic substance.
According to a still further embodiment of the present invention, there is provided an ink jet recording apparatus comprising:
a first ink container containing a black ink; and
a second ink container containing a color ink; and
a head for ejecting each of the black ink and the color ink,
wherein the black ink comprises a pigment having a cationic group or a pigment and a pigment-dispersant having a cationic group, and a resin encapsulating a coloring material, and the color ink comprises an anionic dye and an anionic substance.
According to a still further embodiment of the present invention, there is provided a method for alleviating bleeding in a boundary between a black image formed by an ink-jet method with a black ink and a color image formed by an ink-jet method with a color ink,
wherein the black ink comprises a pigment having a cationic group or a pigment and a pigment-dispersant having a cationic group, and a resin encapsulating a coloring material, and the color ink comprises an anionic dye and an anionic substance.